I don't think any of the big name popstars are necessarily boring. I mean, people who are boring don't become popstars. I suppose if I had to choose which ones were the closest to boring, it would have to be Katy, Adele, and Britney for me.
idg how you could call britney boring, maybe personality wise at times I guess although she's been betetr this era.
IMO Taylor Swift is boring and predictable musically.